Drug-delivery endovascular stent and method of forming the same

ABSTRACT

An intravascular stent and method for inhibiting restenosis, following vascular injury, is disclosed. The stent has an expandable, linked-filament body and a drug-release coating formed on the stent-body filaments, for contacting the vessel injury site when the stent is placed in-situ in an expanded condition. The coating releases, for a period of at least 4 weeks, a restenosis-inhibiting amount of a monocyclic triene immunosuppressive compound having an alkyl group substituent at carbon position 40 in the compound. The stent, when used to treat a vascular injury, gives good protection against clinical restenosis, even when the extent of vascular injury involves vessel overstretching by more than 30% diameter. Also disclosed is a stent having a drug-release coating composed of (i) 10 and 60 weight percent poly-dl-lactide polymer substrate and (ii) 40-90 weight percent of an anti-restenosis compound, and a polymer undercoat having a thickness of between 1-5 microns.

FIELD OF THE INVENTION

The present invention relates to an endovascular drug-delivery stent andto a method for treating restenosis.

BACKGROUND OF THE INVENTION

A stent is a type of endovascular implant, usually generally tubular inshape, typically having a lattice, connected-wire tubular constructionwhich is expandable to be permanently inserted into a blood vessel toprovide mechanical support to the vessel and to maintain or re-establisha flow channel during or following angioplasty. The support structure ofthe stent is designed to prevent early collapse of a vessel that hasbeen weakened and damaged by angioplasty. Insertion of stents has beenshown to prevent negative remodeling and spasm of the vessel whilehealing of the damaged vessel wall proceeds over a period of months.

During the healing process, inflammation caused by angioplasty and stentimplant injury often causes smooth muscle cell proliferation andregrowth inside the stent, thus partially closing the flow channel, andthereby reducing or eliminating the beneficial effect of theangioplasty/stenting procedure. This process is called restenosis. Bloodclots may also form inside of the newly implanted stent due to thethrombotic nature of the stent surfaces, even when biocompatiblematerials are used to form the stent.

While large blood clots may not form during the angioplasty procedureitself or immediately post-procedure due to the current practice ofinjecting powerful anti-platelet drugs into the blood circulation, somethrombosis is always present, at least on a microscopic level on stentsurfaces, and it is thought to play a significant role in the earlystages of restenosis by establishing a biocompatible matrix on thesurfaces of the stent whereupon smooth muscle cells may subsequentlyattach and multiply.

Stent coatings are known which contain bioactive agents that aredesigned to reduce or eliminate thrombosis or restenosis. Such bioactiveagents may be dispersed or dissolved in either a bio-durable orbio-erodable polymer matrix that is attached to the surface of the stentwires prior to implant. After implantation, the bioactive agent diffusesout of the polymer matrix and preferably into the surrounding tissueover a period lasting at least 4 weeks, and in some cases up to 1 yearor longer, ideally matching the time course of restenosis, smooth musclecell proliferation, thrombosis or a combination thereof.

If the polymer is bioerodable, in addition to release of the drugthrough the process of diffusion, the bioactive agent may also bereleased as the polymer degrades or dissolves, making the agent morereadily available to the surrounding tissue environment. Bioerodablestents and biodurable stents are known where the outer surfaces or eventhe entire bulk of polymer material is porous. For example, PCTPublication No. WO 99/07308, which is commonly owned with the presentapplication, discloses such stents, and is expressly incorporated byreference herein. When bioerodable polymers are used as drug deliverycoatings, porosity is variously claimed to aid tissue ingrowth, make theerosion of the polymer more predictable, or to regulate or enhance therate of drug release, as, for example, disclosed in U.S. Pat. Nos.6,099,562, 5,873,904, 5,342,348, 5,873,904, 5,707,385, 5,824,048,5,527,337, 5,306,286, and 6,013,853.

Heparin, as well as other anti-platelet or anti-thrombolytic surfacecoatings, are known which are chemically bound to the surface of thestent to reduce thrombosis. A heparinized surface is known to interferewith the blood-clotting cascade in humans, preventing attachment ofplatelets (a precursor to thrombin) on the stent surface. Stents havebeen described which include both a heparin surface and an active agentstored inside of a coating (see U.S. Pat. Nos. 6,231,600 and 5,288,711,for example).

A variety of agents specifically claimed to inhibit smooth muscle-cellproliferation, and thus inhibit restenosis, have been proposed forrelease from endovascular stents. As examples, U.S. Pat. No. 6,159,488describes the use of a quinazolinone derivative; U.S. Pat. No.6,171,609, the use of taxol, and U.S. Pat. No. 5,176,98, the use ofpaclitaxel, a cytotoxic agent thought to be the active ingredient in theagent taxol. The metal silver is cited in U.S. Pat. No. 5,873,904.Tranilast, a membrane stabilizing agent thought to haveanti-inflammatory properties is disclosed in U.S. Pat. No. 5,733,327.

More recently, rapamycin, an immunosuppressant reported to suppress bothsmooth muscle cell and endothelial cell growth, has been shown to haveimproved effectiveness against restenosis, when delivered from a polymercoating on a stent. See, for example, U.S. Pat. Nos. 5,288,711 and6,153,252. Also, in PCT Publication No. WO 97/35575, the monocyclictriene immunosuppressive compound everolimus and related compounds havebeen proposed for treating restenosis, via systemic delivery.

Ideally, a compound selected for inhibiting restenosis, by drug releasefrom a stent, should have three properties. First, because the stentshould have a low profile, meaning a thin polymer matrix, the compoundshould be sufficiently active to produce a continuous therapeutic dosefor a minimum period of 4-8 weeks when released from a thin polymercoating. Secondly, the compound should be effective, at a low dose, ininhibiting smooth muscle cell proliferation. Finally, endothelial cellswhich line the inside surface of the vessel lumen are normally damagedby the process of angioplasty and/or stenting. The compound should allowfor regrowth of endothelial cells inside the vessel lumen, to provide areturn to vessel homeostasis and to promote normal and criticalinteractions between the vessel walls and blood flowing through thevessel.

SUMMARY OF THE INVENTION

The invention includes, in one aspect, an endovascular stent forplacement at a vascular injury site, for inhibiting restenosis at thesite. The stent is constructed of a structural member or body formed ofone or more filaments and carried on the stent-body filament(s), abioerodable drug-release coating having a thickness of between 3-15microns, and composed of (i) 20 and 60 weight percent poly-dl-lactidepolymer substrate and (ii) 40-80 weight percent of an anti-restenosiscompound. A polymer undercoat having a thickness of between 1-5 micronsand disposed between the stent-body filaments and the coating helps tostabilize the coating on the stent filaments. The stent is expandablefrom a contracted condition in which the stent can be delivered to avascular injury site via catheter, and an expanded condition in whichthe stent coating can be placed in contact with the vessel at the injurysite. The stent coating is effective to release a restenosis-inhibitingamount of the compound over a period of at least 4 weeks after the stentis placed at the vascular injury site.

In various exemplary embodiments, the anti-restenosis compound is amonocyclic triene immunosuppressive compound, the stent body is ametal-filament structure, the undercoat is formed of a parylene polymerand has a thickness between 0.5 and 5 microns, and the coating has athickness between 2 and 10 microns. The compound may be present in thecoating in an amount between 50% and 75% by weight.

Exemplary macrocyclic triene immunosuppressive compounds have thegeneral form

where (i) R is H or CH₂—X—OH, and X is a linear or branched alkyl groupcontaining 1 to 7 carbon atoms, when R′ is H (R′ replaces H at the 28position O) or (ii) at least one of R and R′ have the form

where m is an integer from 1 to 3 and R₁ and R₂ are each a hydrogen, oran alkyl radical having from one to three carbon atoms, or,alternatively, wherein R₁ and R₂ together with a nitrogen atom to whichthey are attached form a saturated heterocyclic ring having four carbonatoms. In an exemplary compound, known as everolimus, R′ is H and X is—CH₂.

The above stent is employed in a method for inhibiting restenosis in avascular injury site, in accordance with another aspect of theinvention. In the method, the stent is delivered to a vascular injurysite, and expanded to bring the stent coating in contact with the vesselat the injury site. The coating is effective to release arestenosis-inhibiting amount of the compound over a period of at least 4weeks.

In another aspect, the invention includes an endovascular stent forplacement at a vascular injury site for inhibiting restenosis at thesite. The stent is composed of a structural member or body formed of oneor more filaments and carried on the stent-body filament(s), adrug-release coating having a thickness of between 3-25 microns, andcomposed of (i) 20 and 70 weight percent polymer substrate and (ii)30-80 weight percent macrocyclic triene immunosuppressive compoundhaving the form:

where R is CH²⁻—X—OH, and X is a linear group containing 1 to 7 carbonatoms.

The stent is expandable from a contracted condition in which the stentcan be delivered to a vascular injury site via catheter, and an expandedcondition in which the stent coating can be placed in contact with thevessel at the injury site. The coating is effective to release therestenosis-inhibiting amount of the compound over a period of at least 4weeks after the stent is placed at the vascular injury site.

In various exemplary embodiments, R is CH₂—X—OH where X is —CH₂—, thestent body is a metal-filament structure, and the polymer substrate inthe coating is a polymethylmethacrylate, ethylene vinyl alcohol, orpoly-dl-lactide polymer.

In one exemplary embodiment, the polymer substrate in the coating isformed of a bioerodable poly-dl-lactide having a thickness between 3-20microns and the compound is present in the coating at an initialconcentration of between 20 and 70 weight percent of coating.Particularly where the amount of the compound in the coating is greaterthan about 40 weight percent, the stent may further include a parylenepolymer undercoat having a thickness of between 1-5 microns, disposedbetween the filaments of the stent body and the poly-dl-lactide coatingsubstrate.

Alternatively, both the stent body and coating substrate may be formedof a bioerodable polymer, such poly-l- or poly-dl-lactide forming thestent-body filaments, and poly-dl-lactide forming the coating substrate.

The stent coating may be constructed to contact blood flowing throughthe stent when the stent is placed at the site in its expandedcondition. In this embodiment, the coating may further contain abioactive agent such as an anti-platelet, fibrinolytic, or thrombolyticagent in soluble crystalline form. Exemplary anti-platelet,fibrinolytic, or thrombolytic agents are heparin, aspirin, hirudin,ticlopidine, eptifibatide, urokinase, streptokinase, tissue plasminogenactivator (TPA), or mixtures thereof.

In still another aspect, the invention provides an improvement in amethod for restenosis at a vascular injury site, by placing at the sitean endovascular stent designed to release a macrocyclic trieneimmunosuppressive compound over an extended period. The improvementincludes employing as the macrocyclic triene immunosuppressive compound,a compound having the formula:

where R is CH²⁻—X—OH, and X is a linear alkyl group containing 1 to 7carbon atoms. In one exemplary compound, X is —CH₂—.

Various exemplary embodiments of the stent composition are given above.

Also disclosed is a novel method for coating the filaments of a stentbody with a drug-containing polymer coating. The method employs anautomated controller to regulate the flow of a polymer or polymer-drugsolution onto the filaments of a stent body, to achieve one of a varietyof stent-coating features, including a uniform thickness coating on oneor more sides of the stent-body filaments, greater coating thickness onthe outer (or inner) surfaces of the stent body than on the other side,inner and outer coatings containing different drugs, and/or coatingthickness gradients or discrete coating patches on the stent body.

These and other objects and features of the invention will become morefully apparent when the following detailed description of the inventionis read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate an endovascular stent having a metal-filamentbody, and formed in accordance with one embodiment of the presentinvention, showing the stent in its contracted (FIG. 1) and expanded(FIG. 2) conditions;

FIG. 3 is an enlarged cross-sectional view of a coated metal filament inthe stent of FIG. 1;

FIG. 4 is an enlarged cross-sectional view of coated erodable polymerstent;

FIGS. 5A and 5B are schematic illustrations of a polymer coating methodsuitable for use in producing the coated stent of the invention;

FIG. 6 shows a bioerodable polymer stent constructed in accordance withthe present invention, and mounted on a catheter for delivery to avascular site;

FIGS. 7A and 7B is are plots showing release of everolimus from stentsconstructed in accordance with the invention;

FIG. 8 is a cross-sectional view of a stent in the invention deployed ata vascular site;

FIGS. 9A-9C are histological sections of a vessel 28 days afterimplantation of a bare-metal stent;

FIGS. 10A-10C are histological sections of a vessel 28 days afterimplantation of a metal-filament stent with a polymer coating;

FIGS. 11A-11C and 12A-12C are histological sections of a vessel 28 daysafter implantation of a metal-filament stent with a polymer coatingcontaining everolimus;

FIG. 13 is an enlarged histological section of a vessel seen with afilament of the stent employed in FIGS. 11A-11C, which been overgrown bynew tissue forming a healed vessel wall;

FIG. 14 is a plot of area of stenosis at 28 days post-implant, as afunction of injury score, with a variety of different stents, includingthose constructed in accordance with the invention; and

FIG. 15 shows a correlation plot between injury score (Y axis) and B/A(balloon/artery) ratio at time of stent implantation.

DETAILED DESCRIPTION OF THE INVENTION I. Endovascular Stent

FIGS. 1 and 2 show a stent 20 constructed in accordance with theinvention, in the stent's contracted and expanded states, respectively.The stent includes a structural member or body 22 and an outer coatingfor holding and releasing an anti-restenosis compound, as will bedescribed further below with reference to FIGS. 3 and 4.

A. Stent Body

In the embodiment shown, the stent body is formed of a plurality oflinked tubular members by filaments, such as members 24, 26. Each memberis has an expandable zig-zag, sawtooth, or sinusoidal wave structure.The members are linked by axial links, such as links 28, 30 joining thepeaks and troughs of adjacent members. As can be appreciated, thisconstruction allows the stent to be expanded from a contractedcondition, shown in FIG. 1, to an expanded condition, shown in FIG. 2,with little or no change in the length of the stent. At the same time,the relatively infrequent links between peaks and troughs of adjacenttubular members allows the stent to accommodate bending. This featuremay be particularly important when the stent is being delivered to avascular site in its contracted state, in or on a catheter. The stenthas a typical contracted-state diameter (FIG. 1) of between 0.5-2 mm,more preferably 0.71 to 1.65 mm, and a length of between 5-100 mm. Inits expanded state, shown in FIG. 2, the stent diameter is at leasttwice and up to 8-9 times that of the stent in its contracted state.Thus, a stent with a contracted diameter of between 0.7 to 1.5 mm mayexpand radially to a selected expanded state of between 2-8 mm or more.

Stents having this general stent-body architecture of linked, expandabletubular members are known, for example, as described in PCT PublicationNo. WO 99/07308, which is commonly owned with the present application,and which is expressly incorporated by reference herein. Furtherexamples are described in U.S. Pat. Nos. 6,190,406, 6,042,606,5,860,999, 6,129,755, or 5,902,317, which patents are incorporated byreference herein. Alternatively, the structural member in the stent mayhave a continuous helical ribbon construction, that is, where the stentbody is formed of a single continuous ribbon-like coil. The basicrequirement of the stent body is that it be expandable, upon deploymentat a vascular injury site, and that it is suitable for receiving adrug-containing coating on its outer surface, for delivering drugcontained in the coating into the vessel wall (i.e. medial, adventitial,and endothelial layers of tissue) lining the vascular target site.Preferably, the body also has a lattice or open structure, allowingendothelial cell wall ingrowth “through” the stent from outside toinside.

B. Stent Coatings

According to an important feature of the invention, the stent filamentsare coated with a drug-release coating composed of a polymer matrix andan anti-restenosis compound (active compound) distributed within thematrix for release from the stent over an at least a several weekperiod, typically 4-8 weeks, and optionally over a 2-3-month period ormore.

FIG. 3 shows, in enlarged sectional view, a stent filament 24 having acoating 32 that covers the filament completely on all sides, that is, ontop (the filament side forming the outer surface of the stent body)bottom (the filament side forming the interior surface of the stent) andthe opposing filament sides. As will be discussed further below, thecoating has a thickness typically between 3 and 30 microns, depending onthe nature of the polymer matrix material forming the coating and therelative amounts of polymer matrix and active compound. Ideally, thecoating is made as thin as possible, e.g., 15 microns or less, tominimize the stent profile in the vessel at the injury site.

The coating should also be relatively uniform in thickness across theupper (outer) surfaces, to promote even distribution of released drug atthe target site. Methods for producing a relatively even coatingthickness on stent filaments are discussed below in Section II.

Also shown in FIG. 3 is a polymer underlayer 34 disposed between thestent filament and the coating. The purpose of the underlayer is to helpbond the coating to the stent-body filaments, that is, to help stabilizethe coating on the filaments. As will be seen below, this function isparticularly valuable where the coating is formed of a polymer substratecontaining a high percentage of anti-restenosis compound, e.g. between35-80 weight percent compound. One exemplary underlayer polymer isparylene used in conjunction with a polymer substrate formed ofbioerodable (poly-dl-lactide). Other suitable polymer underlayers areethylene vinyl alcohol (EVOH), paryLAST™, silicone, TEFLON™ and otherfluoropolymers, that may be deposited on the metal stent surfaces byplasma-coating or other coating or deposition processes. The underlayerhas a typical thickness between 1-5 microns.

The polymer forming the substrate may be any biocompatible polymermaterial from which entrapped compound can be released by diffusionand/or released by erosion of the polymer matrix. Two well-knownnon-erodable polymers for the coating substrate arepolymethylmethacrylate and ethylene vinyl alcohol. Methods for preparingthese polymers in a form suitable for application to a stent body aredescribed for example, in US 2001/0027340A1 and WO00145763A1,incorporated herein by reference. In general, the limit of drug additionto the polymers is about in the range of 20-40 weight percent.

Bioerodable polymers, particularly poly-dl-lactide polymer, are alsosuitable for coating substrate material. In one general embodiment, ofthe invention, the coating is a bioerodable poly-dl-lactide polymersubstrate, i.e., poly-dl-lactic acid polymer, that may contain up to 80%by dry weight of the active compound distributed within the polymersubstrate. More generally, the coating contains 35-80% dry weight activecompound and 20-65% percent by dry weight of the polymer. Exemplarycoatings include 25-50% dry weight polymer matrix and 50-75 weightpercent active compound. The polymer is formulated with the activecompound for deposition on the stent filaments as detailed in Section IIbelow.

A variety, of anti-restenosis compounds may be employed in theembodiment, including anti-proliferative agents, such as taxol,antisense compounds, doxorubicin, and most particularly, macrocyclictriene immunosuppressive compounds having the general structureindicated below. The latter class of compounds, and their synthesis, aredescribed, for example in U.S. Pat. Nos. 4,650,803, 5,288,711,5,516,781, 5,665,772 and 6,153,252, in PCT Publication No. WO 97/35575,and in published U.S. Pat. No. 6,273,913B1, 60/176086, 20000212/17, and2001002935/A1, all of which are incorporated herein by reference.Exemplary macrocyclic triene immunosuppressive compounds have the form:

where (i) R is H or CH²⁻—X—OH, and X is H or is a linear or branchedalkyl group containing 1 to 7 carbon atoms, when R′ is H (R′ replaces Hat the 28 position O) or (ii) at least one of R and R′ have the form

where m is an integer from 1 to 3 and R₁ and R₂ are each a hydrogen, oran alkyl radical having from one to three carbon atoms, or,alternatively, wherein R₁ and R₂ together with a nitrogen atom to whichthey are attached form a saturated heterocyclic ring having four carbonatoms. In an exemplary compound, known as everolimus, R′ is H and X is—CH₂.

One preferred coating is formed of 25-50 weight percent poly-dl-lactidepolymer substrate, and 50-75 weight percent macrocyclic trieneimmunosuppressant compound, having a coating thickness of between 3-15microns. The underlayer is formed of parylene, and has a thicknessbetween 1-5 microns. This embodiment typically contains an amount ofcompound equal to about 15 micrograms drug/mm of stent length.

In another exemplary embodiment, the coating is formed of 15-35 weightpercent of an erodable or non-erodable polymer substrate, and 65-85weight percent of a macrocyclic triene compound. The coating thicknessis preferably 10-30 microns, and the stent may include a 1-5 micronpolymer underlayer, e.g., parylene underlayer. This embodiment typicallycontains an amount of compound equal to about 15 micrograms drug/mm ofstent length. The active compound has the form

where R is CH²⁻—X—OH, and X is a linear alkyl group containing 1 to 7carbon atoms. A preferred compound is everolimus, where X=—CH₂.Compounds in which X is a 2, 3, 4, 5, 6, or 7 carbon alkyl group, eitheralone or in any combination, as well as application of acetate esters ofthe foregoing compounds, including everolimus, are also suitable for theinvention.

The coating may additionally include a second bioactive agent effectiveto minimize blood-related events, such as clotting, that may bestimulated by the original vascular injury, the presence of the stent;or to improve vascular healing at the injury site. Exemplary secondagents include anti-platelet, fibrinolytic, or thrombolytic agents insoluble crystalline form. Exemplary anti-platelet, fibrinolytic, orthrombolytic agents are heparin, aspirin, hirudin, ticlopadine(sp),eptifibatide, urokinase, streptokinase, tissue plasminogen activator(TPA), or mixtures thereof. The amount of second-agent included in thestent coating will be determined by the period over which the agent willneed to provide therapeutic benefit. Typically, the agent will bebeneficial over the first few days after vascular injury and stentimplantation, although for some agents, longer period of release of theagent will be required.

The second agent may be included in the coating formulation that isapplied to the stent-body filaments, according to known methods.

C. Bioerodable Stent

In another general embodiment, both the stent body and polymer coatingare formed of a bioerodable polymer, allowing complete resorption of thestent over time. The stent preferably is an expandable coiled stenthaving a helical-ribbon filament forming the stent body (not shown).Self-expandable coil stents are described in U.S. Pat. No. 4,990,155 forimplantation into blood vessels and are incorporated herein byreference.

A coiled stent, may be formed using a preform with the final expandeddiameter of the preform specified to be slightly larger than theinternal lumen size of the blood vessel to be treated with the coil (3.5mm OD±1 mm would be common for a coronary artery). More generally, thestent may be formed by molding, in its expanded shape, and placed in itscontracted state by twisting around the stent's long axis or forcing thestent radially into a contracted condition for delivery to the bloodvessel when mounted on the tip of a catheter. The stent has a totalthickness preferably between about 100 and 1000 microns, and a totallength of between 0.4 and 10 cm. In fact, an important advantage of abioerodable stent of this type is that relatively long stents, e.g.,over 3 cm in length, can be readily delivered and deployed at a vascularinjury site.

Methods for forming balloon-expandable stents formed of a knitted,bioerodable polymer filament such as poly-l-lactide have been reported(U.S. Pat. No. 6,080,177). A version of the device has also been adaptedto release drugs (U.S. Pat. No. 5,733,327).

A preferred polymer material for forming the stent is poly-l- orpoly-dl-lactide (U.S. Pat. No. 6,080,177). As indicated above, the stentbody and coating may be formed integrally as a single expandablefilament stent having anti-restenosis compound contained throughout.Alternatively, a bioerodable coating may be applied to a preformedbioerodable body, as detailed in Section II below. In the latter case,the stent body may be formed of one bioerodable polymer, such aspoly-l-lactide polymer, and the coating from a second polymer, such aspoly-dl-lactide polymer. The coating, if applied to a preformed stent,may have substantially the same compositional and thicknesscharacteristics described above.

FIG. 4 shows a cross section of a filament, e.g., helical ribbon, in abioerodable stent of the type just described, having separately formedbody and coating. The figures show an internal bioerodable stentfilament 36 coated on all sides with a bioerodable coating 38. Anexemplary coating is formed of poly-dl-lactide and contains between20-40 weight percent anti-restenosis drug, such as a macrocyclic trieneimmunosuppressant compound, and 60-80 weight percent polymer substrate.In another general embodiment, the coating contains 45-75 weight percentcompound, and 25-55 weight percent polymer matrix. Other types ofanti-restenosis compounds, such as listed above, may be employed ineither embodiment.

The bioerodable stent has the unique advantage of treating the entirevessel with one device, either in conjunction with pre-dilitation of thevessel with balloon angioplasty if large obstructions are present, or asa prophylactic implant in patients of high risk of developingsignificant future blockages. Since the stent is fully biodegradable, itdoes not affect the patient's chances for later uncomplicated surgery onthe vessel, as a “full metal jacket,” i.e., a string of drug elutingstents containing metal substrates, does.

A secondary agent, such as indicated above, may be incorporated into thecoating for release from the coating over a desired time period afterimplantation. Alternatively, if a secondary agent is used, it may beincorporated into the stent-body filament if the coating applied to thestent body does not cover the interior surfaces of the stent body. Thecoating methods described below in Section II with respect to ametal-filament stent body are also suitable for use in coating apolymer-filament stent body.

II. Stent Coating Methods

Referring now more particularly to the drawings, FIGS. 5A and 5B areschematic illustrations of the stent coating process according to theinvention. A polymer solution 40 is made by dissolving a polymer in acompatible solvent. At least one anti-restenosis compound, and ifdesired, a secondary agent, is added to the solution, either as asuspension or in solution using the same solvent or a different solvent.The completed mixture is placed in a pressurizable reservoir 42.Connected to the reservoir is a fluid pressurization pump 44.

The pressurization pump may be any source of pressure capable of urgingthe solvent mixture to move at a programmed rate through a solutiondelivery tube 46. The pressure pump 44 is under the control of amicrocontroller (not shown), as is well known in the field of precisiondispensing systems. For example, such a microcontroller may comprise4-Axis Dispensing Robot Model numbers I&J500-R and I&J750-R availablefrom I&J Fisnar Inc, of Fair Lawn, N.J., which are controllable throughan RS-232C communications interface by a personal computer, or precisiondispensing systems such as Automove A-400, from Asymtek, of Carlsbad,Calif. A suitable software program for controlling an RS232C interfacemay comprise the Fluidmove system, also available from Asymtek Inc,Carlsbad, Calif.

Attached to reservoir 42, for example, at the bottom of the reservoir,is a solution delivery tube 48 for delivery of the solvent mixture tothe surface of the stent. The pressurizable reservoir 42 and deliverytube 48 are mounted to a moveable support (not shown) which is capableof moving the solvent delivery tube in small steps such as 0.2 mm perstep, or continuously, along the longitudinal axis of the stent as isillustrated by arrow X1. The moveable support for pressurizablereservoir 42 and delivery tube 46 is also capable of moving the tip(distal end) of the delivery tube closer to the microfilament surface orup away from the microfilament surface in small steps as shown by arrowY1.

The uncoated stent is gripped by a rotating chuck contacting the innersurface of the stent at least one end. Axial rotation of the stent canbe accomplished in small degree steps, such as 0.5 degree per step, toreposition the uppermost surface of the stent structure for coating bythe delivery tube by attachment of a stepper motor to the chuck as iswell known in the art. If desirable, the stent can be rotatedcontinuously. The method of precisely positioning a low volume fluiddelivery device is well known in the field of X-Y-Z solvent dispensingsystems and can be incorporated into the present invention.

The action of the fluid pressurizing pump, X1 and Y1 positioning of thefluid delivery tube, and R1 positioning of the stent are typicallycoordinated by a digital controller and computer software program, suchthat the precisely required amount of solution is deposited whereverdesired on the surfaces of the stent, whereupon the solvent is allowedto escape, leaving a hardened coating of polymer and agent on the stentsurfaces. Typically, the viscosity of the solvent mixture is prepared byvarying the amount of solvent, and it ranges from 2 centipoise to 2000centipoise, and typically can be 300 to 700 centipoise. Alternatively,the delivery tube can be held at a fixed position and, in addition tothe rotation movement, the stent is moved along its longitudinaldirection to accomplish the coating process.

The X-Y-Z positioning table and moveable support may be purchased fromI&J Fisnar. The solution delivery tube preferred dimensions arepreferably between 18-28 gauge stainless steel hypotubes mounted to asuitable locking connector. Such delivery tubes may be obtained from EFDInc of East Providence, R.I. See EFD's selection guide for SpecialPurpose Tips. The preferred tips are reorder #'s 5118-1/4-B through5121-1/4-B “Burr-free passivated stainless steel tips with ¼″ length forfast point-to-point dispensing of particle-filled or thick materials”,reorder #'s 51150VAL-B “Oval stainless steel tips apply thick pastes,sealants, and epoxies in flat ribbon deposits”, and reorder #'s5121-TLC-B through 5125-TLC-B “Resists clogging of cyanoacrylates andprovides additional deposit control for low viscosity fluids. Crimpedand Teflon lined”. A disposable pressurizable solution reservoir is alsoavailable from EFD, stock number 1000Y5148 through 1000Y 5152F. Analternate tip for use with the invention is a glass micro-capillary withan I.D. of about 0.0005 to 0.002 inch, such as about 0.001 inch, whichis available from VWR Catalog No. 15401-560 “Microhematocrit Tubes”, 60mm length, I.D. 0.5-0.6 mm.

The tubes are further drawn under a Bunsen burner to achieve the desiredI.D. for precise application of the polymer/drug/solvent mixture. Theprogrammable microcontroller to operate the stepper motor, and XYZ tableis available from Asymtek, Inc. It is within the scope of the inventionto use more than one of the fluid dispensing tube types working inconcert to form the coating, or alternately to use more than onemoveable solution reservoir equipped with different tips, or containingdifferent viscosity solutions or different chemical makeup of themultiple solutions in the same process to form the coating. The chuckand stepper motor system may be purchased from Edmund Scientific ofBarrington, N.J.

Typically, as described above, the coating is applied directly onto theoutside support surface(s) of the stent, and may or may not cover theentire or a portion(s) of the inside surface(s) of the stent dependingon how control is applied to the above described coating system of thepresent invention, as illustrated in FIGS. 5A and 5B. The latter figureshows application of a coating material 52 to top and side regions of afilament 50. Alternatively, the coating or coating mixture can also beapplied directly onto the inside surface of the stent. A thin deliverytip may penetrate through one or more of the cut out areas (i.e.windows) in the wall of the stent structure, and thereby apply thecoating mixture directly onto the inside surfaces at desired areas. Inthis method, it is possible to apply different coating materials havingdifferent drug components to outer and inner sides of the filaments. Forexample, the coating on the outer filament surfaces could contain ananti-restenosis compound, and the coating of the inner filamentsurfaces, one of the above secondary agents, such an anti-thrombotic oranti-clotting compound. If the stent has a large enough diameter, a thin“L-shaped” delivery tip can be inserted into the stent open ends alongthe longitudinal axis of the stent for the purpose of applying coatingto the inside surfaces.

The polymer for use in the invention includes, but is not limited to,poly(d, l-lactic acid), poly(l-lactic acid), poly(d-lactic acid),ethylene vinyl alcohol (EVOH), ε-caprolactone, ethylvinyl hydroxylatedacetate (EVA), polyvinyl alcohol (PVA), polyethylene oxides (PEO), andco-polymers thereof and mixtures thereof, dissolved in chloroform, oracetone, or other suitable solvents. These polymers all have a historyof safe and low inflammatory use in the systemic circulation.

A non-polymer coating such as everolimus which has been ionically boundto the metal stent surface can also be used in the present invention.

Using the coating system as described, it has been discovered that it isfeasible to coat all of the top, side, and inside surfaces of the stent.By the careful selection of a suitable ratio of solvent to polymer, theviscosity of the solution can be adjusted such that some of the solutionwill migrate down the sides of the strut and actually inhabit the bottomsurface before solidifying, as shown in FIG. 5B. By controlling thedwell time of the delivery tube close to the edge of the stent, theamount of polymer coating the edges or bottom of the stent can beincreased or reduced. In the embodiment illustrated in FIG. 3, anunderlayer of pure polymer 34 and solvent is applied to the stentsurfaces 24 first using the coating system of the invention and thesolvent is allowed to evaporate. Then a second layer of polymer 32 isapplied containing the bioactive agent.

As noted above, a secondary agent may be incorporated into the polymermixture. As an example, heparin in crystalline form may be incorporatedinto the coating. The heparin crystals are micronized to a particle sizeof approximately 1-5 microns and added in suspension to the polymersolution. Suitable forms of heparin are those of crystalline form thatexhibit bioactivity in mammalian hosts when applied according to theprocess of the invention, including heparin salts (i.e. sodium heparinand low molecular weight forms of heparin and their salts). Upondeployment of the drug delivering stent into the vessel wall 60, as seenin FIG. 8, the heparin crystals 62 near the surface of the coating ofcured polymer 66 begin to dissolve, increasing the porosity of thepolymer. As the polymer slowly dissolves, more heparin and bioactiveagent are released in a controlled manner, as indicated by 68.

It should be appreciated however, with reference to FIG. 8, that it isnot always desirable to coat the inside surfaces of the stent (indicatedas 61 in FIG. 8). For example, coating the inside surface of the stentincreases the crimped delivery profile of the device, making it lessmaneuverable in small vessels. And, after implantation, the insidesurfaces are directly washed by the flow of blood through the stent,causing any drug released on the inside surface to be lost to thesystemic circulation. Therefore, in the embodiments shown in FIGS. 3 and4, the bulk of the cured polymer and agent is deployed on the outsidecircumference of the stent supports, and secondarily on the sides. In apreferred embodiment, only a minimum amount of polymer and agent isapplied on the inside surfaces of the stent. If desired, it is alsopossible to have at least a portion of the inside surfaces of the stentuncoated or exposed.

Further, the coating of FIGS. 3 and 4, may be placed onto the stentfilament surfaces in a selective manner. The depth of the coated sectionmay correspond to the volume of bioactive coating to be available forpresentation to the tissue. It may be advantageous to restrict thecoating from certain areas, such as those which could incur high strainlevels during stent deployment.

A uniform underlayer may be first placed on the stent surface to promoteadhesion of the coating that contains the bioactive agent, and/or tohelp stabilize the polymer coating on the stent. The primer coat may beapplied by using any of the methods as already known in the art, or bythe precision dispensing system of the invention. It is also within thescope of the invention to apply a primer coat using a different polymermaterial, such as parylene (poly(dichloro-para-xylene)), or any othermaterial which exhibits good adhesion to both the base metal substrateand the coating which contains the bioactive agent. Parylene(poly(dichloro-para-xylene)) may be deposited via sputter coating orvapor deposition techniques as is well known in the art (See U.S. Pat.No. 6,299,604). In one embodiment of the present invention, islands or alayer of a coating containing heparin are formed on inside surface(s) ofa stent and an anti-proliferation coating containing the drugs of thepresent invention as described above is formed on outside surface(s) ofthe stent.

Where it is desired to form a coating with a high drug/polymer substrateratio, e.g., where the drug constitutes 40-80 weight percent of thecoating on a metal stent substrate, it is advantageous to form anunderlayer on the stent filaments to stabilize and firmly attach thecoating to the substrate. The underlayer may be further processed, priorto deposition of the coating material, by swelling in a suitablesolvent, e.g., acetone, chloroform, xylene, or mixtures thereof. Thisapproach is described in Example 5 for preparing a stent having a highratio of everolimus to poly-dl-lactide.

Here a parylene underlayer is formed on the stent filaments by plasmadeposition, and the underlayer then allowed to swell in xylene prior tofinal deposition of the coating material. The method was effective inproducing coating containing 50% drug in one case and 75% drug inanother case in a poly-dl-lactide polymer substrate, in a coating havinga thickness of only 5-10 microns.

It is also within the scope of the present invention to produce acompletely bioerodable stent, as noted above, using the coating systemof the current invention. This may be accomplished by making a tubularpreform in the shape of the stent to be formed, using an open-top“C-shaped” helical channel into which the dispensing system may depositthe polymer. The preform is open at its outside diameter so that thepolymer may be deposited into the preform, typically using one pass, butalso multiple passes, if necessary, of the dispensing tube; whilecreating uniform edges of the stent structure where the polymer isconstrained by the preform. The preform is soluble in a solvent whichdoes not dissolve the bio-degradable stent thus created. After thepolymer has been deposited and solvent of the polymer solution hasevaporated, the assembly may be placed in the solvent which dissolvesthe preform to free the completed stent structure. A typical materialfor the preform is sucrose, which may be molded into the desired preformshape using standard injection molding techniques. A typical solvent forthe preform is water.

III. Methods of Use and Performance Characteristics

This section describes vascular treatment methods in accordance with theinvention, and the performance characteristics of stents constructed inaccordance with the invention.

A. Methods

The methods of the invention are designed to minimize the risk and/orextent of restenosis in a patient who has received localized vascularinjury, or who is at risk of vascular occlusion. Typically the vascularinjury is produced during an angiographic procedure to open a partiallyoccluded vessel, such as a coronary or peripheral vascular artery. Inthe angiographic procedure, a balloon catheter is placed at theocclusion site, and a distal-end balloon is inflated and deflated one ormore times to force the occluded vessel open. This vessel expansion,particularly involving surface trauma at the vessel wall where plaquemay be dislodged, often produces enough localized injury that the vesselresponds over time by cell proliferation and reocclusion. Notsurprisingly, the occurrence or severity of restenosis is often relatedto the extent of vessel stretching involved in the angiographicprocedure. Particularly where overstretching is 35% or more, restenosisoccurs with high frequency and often with substantial severity, i.e.,vascular occlusion.

In practicing the present invention, the stent is placed in itscontracted state typically at the distal end of a catheter, eitherwithin the catheter lumen, or in a contracted state on a distal endballoon. The distal catheter end is then guided to the injury site, orthe site of potential occlusion, and released from the catheter, e.g.,by using a trip wire to release the stent into the site, if the stent isself-expanding, or by expanding the stent on a balloon by ballooninflation, until the stent contacts the vessel walls, in effect,implanting the stent into the tissue wall at the site.

FIG. 6 shows an embodiment of a completely biodegradable stent of thepresent invention with a delivery catheter suitable for implantation ofthe device in a blood vessel of the cardiovascular system, for example acoronary artery, The drawing shows the stent 53, referred to by theinventors as a “Drug Coil”, in a partially released position. The stent,which is a self-expanding coil type, is formed from polylactic acid andcontains one or more active biological agents of the present invention.

The coil is created using a preform as described, with the finalexpanded diameter of the preform specified to be slightly larger thanthe internal lumen size of the vessel to be treated with the coil. Afterremoving the preform, the Drug Coil is wound down by twisting the endsin opposite directions into a coil of smaller radius and thuslycompressed along its entire length down under a slideable sheath to adelivery diameter is approximately ⅓ of its final expanded diameter atbody temperature. The Drug Coil is thin enough in thickness(approximately 25-125 microns) to be readily bent in a tighter radius toform a compressed coil at the Internal diameter of the sheath. Thesheath is slideably disposed on a delivery catheter 55 suitable fordelivery of the stent in its compressed state to the target vessel.Sheath 54 has a gripping means 56 at its proximal end by which theangioplasty operator may pull back the sheath and fully release the DrugCoil when the tip of the delivery catheter is in position in the vessel.

The center of the delivery catheter 55 has a lumen of approximately0.014″ diameter, in which a guidewire 57 having a flexible tip 58 may beslideably disposed. The delivery catheter further has a luer hub 59 forconnection of the inner lumen to a Y-connector and hemostasis valve, asis well known in the angioplasty art. The OD of the delivery catheterwith slideable sheath may be in the range of 2-4 F. (French size), orlarger if peripheral arteries are being treated.

Since the Drug Coil is fully biodegradable, it does not affect thepatients' chances for later uncomplicated surgery on the vessel, as afull metal jacket does. While bare metal coils are often placed invessels to create thromboembolism and complete blockage in certainneurovascular applications, surprisingly it has been determined that thebiocompatible polymer, poly (dl-Lactic) acid (PDLA), and mixturesthereof, in the disclosed configuration provide adequate mechanicalstrength to support the injured vessel following angioplasty, andfurther do not create embolism and thus are exemplary materials formanufacture of Drug Coils of the present invention,

Once deployed at the site, the stent begins to release active compoundinto the cells lining the vascular site, to inhibit cellularproliferation. FIG. 7A shows everolimus release kinetics from two stentsconstructed in accordance with the invention, each having anapproximately 10 micron thick coating (closed squares). Drug-releasekinetics were obtained by submerging the stent in a 25% ethanolsolution, which greatly accelerates rate of drug release from the stentcoating. The graphs indicate the type of drug release kinetics that canbe expected in vivo, but over a much longer time scale.

FIG. 7B shows drug release of everolimus from coatings of the presentinvention on metal stent substrates. The upper set of curves show drugrelease where the coating has been applied directly to the metalsurface. The lower set of curves (showing slower release) were obtainedby applying an underlayer or primer coat of parylene to the metal stentsurface, followed by coating of the surface with the coating system ofthe invention. As seen, the primer increases the mechanical adhesion ofthe coating to the sent surface, resulting in slower breakdown of thebioerodeable coating and slower release of drug. Such a configuration isuseful where it desired to have a strongly attached stent coating whichcan withstand repeated abrasions during tortuous maneuvering of the drugeluting stent inside the guide catheter and/or vessel, and/or where itis desired to slow down the drug release for extended treatment of theatherosclerosis disease process at the implant site followingimplantation of the device.

FIG. 8 shows in cross-section, a vascular region 60 having an implantedstent 62 whose coated filaments, such as filament 64 with coating 66,are seen in cross section. The figure illustrates the release ofanti-restenosis compound from each filament region into the surroundingvascular wall region. Over time, the smooth muscle cells forming thevascular wall begin to grow into and through the lattice or helicalopenings in the stent, ultimately forming a continuous inner cell layerthat engulfs the stent on both sides. If the stent implantation has beensuccessful, the extent of late vascular occlusion at the site will beless than 50%, that is, the cross-sectional diameter of flow channelremaining inside the vessel will be at least 50% of expanded stentdiameter at time of implant.

Trials in a swine restenosis animal model as generally described bySchwartz et al (“Restenosis After Balloon Angioplasty—A PracticalProliferative Model in Porcine Coronary Arteries”, Circulation 82:(6):2190-2200, December 1990.) demonstrate the ability of the stent ofthis invention to limit the extent of restenosis, and the advantages ofthe stent over currently proposed and tested stents, particularly incases of severe vascular injury, i.e., greater than 35% vesselstretching. The studies are summarized in Example 4.

Briefly, the studies compare the extent of restenosis at 28 daysfollowing stent implantation, in bare metal stents, polymer-coatedstents, and polymer coated stents containing high or low concentrationsof sirolimus (rapamycin) and everolimus.

Table 1 in Example 4 shows that both rapamycin (Rapa-high or Rapa-low)and everolimus stents (C-high or C-low) greatly reduced levels ofrestenosis, with the smallest amount of restenosis being observed in thehigh-dose everolimus stent. Similar results were obtained in studies onanimals with low injury (Table 2).

FIGS. 9A-9C are examples of stent cross-sections of neointimal formationat 28 days in a bare metal S-Stent (available from BiosensorsInternational Inc, Newport Beach, Calif.). FIGS. 10A-10C are examples ofneointimal formation in a polymer-coated (no drug) S-Stent; and FIGS.11A-11C and 12A-12C of neointimal formation in everolimus/polymer coatedstents. In general, the vessels with everolimus-coated stent treatmentappeared to be well-healed with a well established endothelial layer,evidence of complete healing and vessel homeostasis at 28 days. FIG. 13is an example of vessel cross-section at 91× magnification showinghealing and establishment of an endothelial layer on the inside of thevessel lumen at 28 days post implant. The photographs indicate that themost favorable combination for elimination of restenosis at 28 days isthe C-high, or C-Ulight formulation (see Example 4), which contained 325microgram and 275 microgram dosage of everolimus on a 18.7 mm lengthstent. The data predicts a 50% reduction in restenosis compared to acurrently marketed bare metal stent (the S-Stent) at 28 days followup inoutbred juvenile swine. The data also shows that the drug everolimus isbetter than, or at least equivalent to the 180 microgram dosage ofsirolimus on the same stent/polymer delivery platform. These results aresupported by morphometric analysis (Example 4).

FIG. 15 shows the relationship between balloon overstretch of thevessel, as measured by balloon/artery ration (B/A Ratio), and vesselinjury in the animal experiment. This data shows that use of anover-expanded angioplasty balloon to create a high controlled vesselinjury is a reasonably accurate method of creating a predictable andknown vascular injury in the porcine model.

FIG. 14 are “best fit” linear regression curves of the chosen dosings ofagents in polymers, coated on the S-Stent, relating injury score to areastenosis at follow-up. Area Stenosis is an accurate indicator ofneointimal formation which is determined by morphometric analysis. Ascan be seen from this chart, the high everolimus stent was the onlycoating in the group of samples tested that exhibited a negative slopevs. increasing injury score. This analysis suggests that the C-highcoating may be capable of controlling restenosis in an injured coronaryartery which is virtually independent of injury score. None of the othercoating formulations tried exhibited this unique characteristic.

From the foregoing, it can be seen how various objects and features ofthe invention are met. In one aspect, the invention provides abioerodable stent coating with high drug/polymer ratios, e.g., 40-80%drug by weight. This feature allows continuous delivery of ananti-restenosis compound over an extended period from a low-profilestent. At the same time, the total amount of polymer breakdowncomponents such as lactide and lactic acid released during bioerosion isrelatively small, minimizing possible side effects, such as irritation,that may result from bioerosion of the stent coating.

In another aspect, the invention provides an improved method fortreating or inhibiting restenosis. The method, which involves a novelcombination of macrocyclic triene immunosuppressant compound in a stentpolymer coating, provides at least the effectiveness against restenosisas the best stent in the prior art, but with the added advantage overthe prior art that the efficacy of the method appears to be independentof the extent of injury, and the method may offer a greater degree ofendothelialization of the stented vessel.

Finally, the method provides a completely bioerodable stent that has theadvantageous features just mentioned and the more design flexibilitythan a metal-body stent, particularly in total stent length and futureoperability on the treated vessel.

The following examples illustrate various aspects of the making andusing the stent invention herein. They are not intended to limit thescope of the invention.

Example 1 Preparation of Everolimus and Derivatives Thereof Step A.Synthesis of 2-(t-butyldimethylsilyl)oxyethanol(TBS glycol)

154 ml of dry THF and 1.88 g NaH are stirred under in a nitrogenatmosphere in a 500 ml round bottom flask condenser. 4.4 mL dry ethyleneglycol are added into the flask, resulting in a large precipitate after45 minutes of stirring. 11.8 g tert-butyldimethylsilyl chloride is addedto the flask and vigorous stirring is continued for 45 minutes. Theresulting mixture is poured into 950 mL ethylether. The ether is washedwith 420 mL brine and solution is dried with sodium sulfate. The productis concentrated by evaporation of the ether in vacuo and purified byflash chromatography using a 27×5.75 cm column charged with silica gelusing a hexanes/Et₂O (75:25 v/v) solvent system. The product is storedat 0° C.

Step B. Synthesis of 2-(t-butyldimethylsilyl)oxyethyl triflate (TBSglycol Trif)

4.22 g TBS glycol and 5.2 g 2,6-lutidine are combined in a double-necked100 mL flask with condenser under nitrogen with vigorous stirring. 10.74g of trifluoromethane sulfonic anhydride is added slowly to the flaskover a period of 35-45 minutes to yield a yellowish-brown solution. Thereaction is then quenched by adding 1 mL of brine, and the solutionwashed 5 times in 100 mL brine to a final pH value of between 6-7. Thesolution is dried using sodium sulfate, and concentrated by evaporationof the methylene chloride in vacuo. The product is purified using aflash chromatography column of approximately 24×3 cm packed with silicagel using hexane/Et₂O (85:15 v/v) solvent system, then stored at 0° C.

Step C. Synthesis of 40-0-[2-(t-butyldimethylsilyl)oxy]ethyl-rapamycin(TBS Rap)

400 mg rapamycin, 10 mL of toluene, and 1.9 ml 2,6-lutidine are combinedand stirred in a 50 mL flask maintained at 55-57 deg C. In a separate 3mL septum vial, 940 μl 2,6-lutidine is added to 1 mL toluene, followedby addition of 2.47 g TBS glycol Trif. The contents of the vial areadded to the 50 mL flask and the reaction allowed to proceed for 1.5hours with stirring. 480 μl 2,6-lutidine plus an additional 1.236 g TBSglycol Trif is added to the reaction flask. Stirring is continued for anadditional hour. Finally, a second portion of 480 μl 2,6-lutidine and1.236 g TBS glycol Trif is added to the mixture, and the mixture isallowed for an additional 1-1.5 hours. The resulting brown solution ispoured through a porous glass filter-using vacuum. The crystal likeprecipitate is washed with toluene until all color has been removed. Thefiltrate is then washed with 60 mL saturated NaHCO₃ solution twice andthen washed again with brine. The resulting solution is dried withsodium sulfate and concentrated in vacuo. A small quantity of ahexane/EtOAc (40:60 v/v) solvent is used to dissolve the product, andpurification is achieved using a 33×2 cm flash chromatography columnpacked with silica gel, and developed with the same solvent. The solventis removed in vacuo and the product stored at 5° C.

Step D. Synthesis process of 40-0-(2-hydroxyl)ethyl-rapamycin(everolimus)

A pyrex glass dish (150×75 mm) is filled with ice and placed on astirring plate. A small amount of water is added to provide an iceslurry. 60-65 mg of TBS-Rap is first dissolved in a glass vial by adding8 ml methanol. 0.8 ml 1N HCl is added to the vial, the solution isstirred for 45 minutes and then neutralized by adding 3 mL aqueoussaturated NaHCO₃. 5 mL brine is added to the solution, followed with 20mL EtoAc, resulting in the formation of two phases. After mixing of thephases, a separatory funnel is used to draw off the aqueous layer. Theremaining solvent is washed with brine to a final pH of 6-7, and driedwith sodium sulfate. The sodium sulfate is removed using a porous glassfilter, and the solvent removed in vacuo. The resulting concentrate isdissolved in EtoAc/methanol (97:3) and then purified using in a 23×2 cmflash chromatography column packed with silica gel, and developed usingthe same solvent system. The solvent is removed in-vacuo and the productstored at 5° C.

Example 2 Preparation of Stent Containing Everolimus in aPoly-dl-lactide Coating

100 mg poly (dl-lactide) was dissolved into 2 mL acetone at roomtemperature. 5 mg everolimus was placed in a vial and 400 μL lactidesolution added. A microprocessor-controlled syringe pump was used toprecision dispense 10 μL of the drug containing lactide solution to thestent strut top surfaces. Evaporation of the solvent resulted in auniform, drug containing single polymer layer on the stent.

A 15 μL volume was used in a similar manner to coat the stent top andside strut surfaces, resulting in a single layer coating on the stentstrut top and sides.

Example 3 In Vitro Drug Release from Stent Containing Everolimus in aPoly-dl-lactide Coating

In vitro drug release was conducted by placing the coated stents into 2mL pH 7.4 phosphate buffered saline solution containing 25% ETOH, andpreserved with 0.05% (w/v) sodium azide and maintained at 37° C.Sampling was periodically conducted by withdrawing the total buffervolume for drug measurement while replacing solution with a similarvolume of fresh buffer (infinite sink). FIG. 7 illustrates drug releasefrom two similar stents coated with a single polymer layermicrodispensed in this manner.

Example 4 Animal Implant Tests

A. QCA Results of Safety and Dose-Finding Studies in Swine Rationale:

It was reasoned that the most challenging treatment condition for thedrug eluting stent is a severely injured vessel, as it is known that thedegree of restenosis (neointimal formation) increases directly withextent of vessel injury. Experiments were conducted in pigs, and asubstantial number of the vessels which were the target of drug-coatedstent implants were seriously injured (averaging approximately 36%overstretch injury of the vessel) using an angioplasty balloon. Thiscaused severe tearing and stretching of the vessel's intimal and mediallayers, resulting in exuberant restenosis at 28 days post implant. Inthis way, it was possible to assess the relative effectiveness ofvarious dosings of drug, and drug to polymer weight ratios on the samemetal stent/polymer platform for reduction of restenosis at 28 dayspost-implant.

DEFINITIONS

-   -   1. Bare stent: An 18.7 mm bare metal stent of a corrugated ring        design (i.e. a currently marketed “S-Stent” as manufactured by        Biosensors Intl., Inc).    -   2. C-high: An 18.7 mm long stent carrying 325 micrograms of        everolimus in a PDLA (poly-dl-lactate) polymer coating.    -   3. C-low: An 18.7 mm long stent carrying 180 micrograms of        everolimus in a PDLA polymer coating.    -   4. Rap-high: An 18.7 mm long stent carrying 325 micrograms of        sirolimus in a PLA polymer coating.    -   5. Rap-low: An 18.7 mm long stent carrying 180 micrograms of        sirolimus in a PDLA polymer coating.    -   6. C-Ulight: An 18.7 mm long stent carrying 275 micrograms of        everolimus in an ultrathin coating of PDLA polymer (37% drug to        polymer weight ratio)    -   7. C-Ulow: An 18.7 mm long stent carrying 180 micrograms of        everolimus or equivalent in an ultrathin coating of PDLA polymer        (37% drug to polymer weight ratio)    -   8. Polymer stent: An 18.7 mm S-Stent stent covered by PDLA        polymer coating only.    -   9. B/A is the final inflated balloon-to-artery ratio, an        indication of the extent of overstretching of the vessel.    -   10. Mean Lumen Loss (MLL)—Average of 3 measurements taken inside        the stent internal lumen at time of implant minus average of 3        measurements at follow-up angiography indicates the amount of        neointima that has formed inside the stent.

Methods:

Drug-eluting stents using a metal wire-mesh scaffold of a corrugatedring design (i.e. S-Stent) and polymer coating were implanted inout-bred juvenile swine (alternately Yucatan Minipigs for implantstudies lasting longer than 28 days), using different dosings of eitherthe drug everolimus or the drug sirolimus. At time of implant,Quantitative Coronary Angiography (QCA) was performed to measure thediameter of the vessels both before and after stent implantation. At 28days, or longer when specified in the table below, the animals wereagain subjected to QCA in the area of the stent, prior to euthanization.

Following euthanasia of animals according to approved protocols, thehearts were removed from the animals and pressurized formaldehydesolution was infused into the coronary arteries. The coronary segmentscontaining the stents were then surgically removed from the surface ofthe heart and subsequently fixed in acrylic plastic blocks fortransverse sectioning with a diamond saw. 50 micron thick sections ofthe acrylic material containing cross-sections of the vessels locatedproximally, center, and distally were then optically polished andmounted to microscope slides.

A microscope containing a digital camera was used to generate highresolution images of the vessel cross-sections which had been mounted toslides. The images were subjected to histomorphometric analysis by theprocedure as follows:

A computerized imaging system Image Pro Plus 4.0 through an A.G. Heinzeslide microscope for a PC-based system was used for histomorphometricmeasurements of:

-   -   1. The mean cross sectional area and lumen thickness (area        circumscribed by the intima/neointimal-luminal border);        neointimal (area between the lumen and the internal elastic        lamina, IEL, and when the IEL was missing, the area between the        lumen and the remnants of media or the external elastic lamina,        EEL); media (area between the IEL and EEL); vessel size (area        circumscribed by the EEL but excluding the adventitial area);        and adventitia area (area between the periadventitial tissues,        adipose tissue and myocardium, and EEL).    -   2. The injury score. To quantify the degree of vascular injury,        a score based on the amount and length of tear of the different        wall structures was used. The degree of injury was calculated as        follows:        -   0=intact IEL        -   1=ruptured IEL with exposure to superficial medial layers            (minor injury)        -   2=ruptured IEL with exposure to deeper medial layers (medial            dissection)        -   3=ruptured EEL with exposure to the adventitia.

The following table shows the results of the QCA analysis (measurementsof mean late loss due to restenosis) at follow-up QCA. The data in thetables below under column heading “Neo-intimal area” report the resultsof morphometic analysis of stents and vessels removed from the pigs atfollow-up (f/u):

TABLE 1 Results of “high injury” experiment Mean Neo- B/A Lumen IntimaDevice Ratio Days Loss, mm Area Description (avg) f/u (avg) (mm²) Stentnumbers Bare Metal 1.33 28 1.69 5.89 31, 39, 40, 45, Stent 47, 50Polymer 1.36 28 2.10 5.82 32, 41, 43, 48, Coated 51, 60 Rapa-high 1.3928 1.07 3.75 42, 44, 49, 65, 69, 73 Rapa-low 1.42 28 0.99 2.80 52, 56,61, 64, 68, 72 C-high 1.37 28 0.84 3.54 54, 55, 59, 63 C-low 1.36 28 154 3.41 53, 57, 58, 62, 66, 70, 74 C-Uhigh 1.36 28 0.85 2.97 67, 75, 92,103

B. Low-Injury Studies

To further determine which dosage of everolimus would be best in alightly injured vessel, more typical of the patient with uncomplicatedcoronary disease and a single denovo lesion, the everolimus elutingstents were implanted to create moderate to low overstretch injury(approximately 15%). Farm swine were used for a 30 day experiment, andadult Yucatan minipigs were implanted for a 3 month safety study. Theangiographic results were as follows:

TABLE 2 QCA Results of “low injury” experiments Neo- Days Mean IntimaDevice B/A post Lumen Area Description ratio implant Loss (mm²) Stentnumbers Bare Metal 1.14 28 0.95 2.89 20, 22, 26, 29 Stent Bare Metal1.13 90 76, 80, 84, 87, 91 Stent C-Uhigh 1.15 28 0.60 2.14 94, 96, 98,102 C-Ulow 1.09 28 0.49 2.26 93, 95, 97, 100, 101 C-Uhigh 1.15 90 77,81, 85, 86, 90

The above data predict that with either the C-Ulow or C-Uhigh doses ofeverolimus will produce a 45-48% reduction in neointimal formation in alow to moderately injured vessel.

C. Morphometric Analysis The total cross-sectional area inside eachstent, and cross-sectional area of new tissue (neo-intima) that hadformed inside the stent were measured by computer, and the % Areastenosis computed. The average vessel injury score, neo-intimal area,and % area stenosis for each formulation of drug and polymer, averaging3 slices per stent, is shown in the table below.

TABLE 3 Results of “high injury” experiment Neo- Intimal Device InjuryDays Area % Area Description Score f/u (mm²) Stenosis Stent numbers BareMetal 1.9 28 5.89 0.72 31, 39, 40, 45, Stent 47, 50 Polymer 2.11 28 5.820.70 32, 41, 43, 48, Coated 51, 60 Rapa-high 2.10 28 3.75 0.55 42, 44,49, 65, 69, 73 Rapa-low 1.90 28 2.80 0.43 52, 56, 61, 64, 68, 72 C-high1.89 28 3.54 0.38 54, 55, 59, 63 C-low 2.1 28 3.41 0.53 53, 57, 58, 62,66, 70, 74 C-Uhigh 2.13 28 2.97 0.45 67, 75, 92, 103

Discussion:

Morphometric analysis is considered a highly accurate method ofmeasuring in-stent restenosis in the pig coronary model. In the highinjury model, the C-High formulation produced the lowest amounts ofneointima formation in the High Injury Experiment at 28 days, however,the C-Uhigh had the highest injury score of the group, and still manageda very low % Area Stenosis of 0.45. Therefore, the data independentlyconfirm the findings of the QCA analysis, and supports the choice ofC-Uhigh as the preferred formulation for human trials.

D. Histological Analysis

The slides for the C-Uhigh and Sirolimus Low were submitted to anexperienced cardiac pathologist, who reviewed the vessel cross-sectionsfor evidence of inflammation, fibrin, and endothelialization of thenewly healed vessel lumen. No difference was found between thehistological changes caused by the sirolimus and everolimus elutingstents. In general, the vessels appeared to be well-healed with a wellestablished endothelial layer, evidence of complete healing and vesselhomeostasis at 28 days. FIG. 13 is an example of vessel cross-section at91× magnification showing healing and establishment of an endotheliallayer on the inside of the vessel lumen at 28 days post-implant.

E. Comparison to Published Results

Carter et.al. have published results of sirolimus-coated stents usingthe Palmaz Schatz metal stent in swine. A table comparing the publishedresults of Carter to Biosensors' experimental results is shown below:

TABLE 4 Mean Std Neointima Late Devia- Cross- Vessel Loss tion SectionalDEVICE Overstretch (mm) (mm) Area (mm²) DESCRIPTION % mm mm mm² S-StentBARE 33.5% +− 9.2% 1.80 +−0.5 7.6 METAL control S-Stent Polymer- 34.9%+− 4.8% 2.02 +−0.8 8.5 only Coated S-Stent 32.9% +− 10.1% 0.66 +−0.2 3.27 Polymer/Rapamycin (−57% vs 325 microGrams control) S-StentPolymer/ 36.8% +− 8.5% 0.74 +−0.3  3.61 Drug #1 (−50% vs 325 microGramscontrol) PS Stent BARE* 10-20% 1.19 — 4.5 control PS Stent Polymer-10-20% 1.38 — 5.0 only PS Rapamycin- 10-20% 0.70 — 2.9 eluting stent*166 (−35.5% microGrams vs control) PS Rapamycin- 10-20% 0.67 — 2.8eluting (−37.7% vs Stent* 166 control) microGrams (Slow Release) PSRapamycin- 10-20% 0.75 — 3.1 eluting (−31.1% vs Stent* 450 control)microGrams

Example 5 Preparation of Stent with High Drug Loading

As-marketed metal corrugated-ring stents (“S-stent, corrugated ringdesign, Biosensors Intl), 14.6 mm in length, were coated with anapproximately 2 micron thick layer of parylene ‘C’ primer coating usinga plasma deposition process.

Parylene coated stents were placed in xylene overnight at ambienttemperature. A stock poly(D,L)—lactic acid solution containing 50 ug/uLpolylactic acid (PLA) was prepared by dissolving 100 mg PLA in 2 mLacetone.

To prepare stents containing a drug to polymer ratio of 50%, 5 mgeverolimus was dissolved in 100 μL of the PDLA stock solution. Anadditional 20 μL acetone was added to aid in dispensing the solution.The stents were removed from the xylene and carefully blotted to removesolvent. A total of 5.1 μL coating solution was dispensed onto the outersurface of each stent. The stents were dried at ambient temperature andplaced into overnight desiccation. This resulted in a total of 212 μgeverolimus contained in 212 μg PLA per stent.

To prepare stents containing a drug to polymer ratio of 75%, 5 mgeverolimus and 33.3 μL stock PLA solution were mixed. An additional 33.3μL acetone was added and the mixture was dissolved. Stents were removedfrom the xylene and blotted similar to above. A total of 2.8 μL coatingsolution was dispensed onto the outer surface of each stent. The stentswere dried at ambient temperature and placed into overnight desiccation.This resulted in a total of 212 μg everolimus contained in 70 μg PLA perstent.

The finished stents exhibited an approximately 5 microns-thick coatingof everolimus/PDLA, or slightly milky appearance, which was smoothlydistributed on the top and side surfaces, and firmly attached to themetal strut surfaces.

It is claimed:
 1. An endovascular stent for placement at a vascularinjury site, for inhibiting restenosis at the site, comprising a bodyhaving an open-lattice structure formed of linked filaments saidfilaments having outside, side and inside surfaces, and carried on theone or more filaments a drug-release coating of a poly-lactide polymerhaving a thickness of between 3-30 microns, and composed of (i) 25-55weight percent polymer substrate and (ii) 45-75 weight percentmacrocyclic triene compound having the form:

where R is CH₂—CH₂—OH or OH, said stent being expandable from acontracted condition in which the stent can be delivered to a vascularinjury site via catheter, and an expanded condition in which the stentcoating can be placed in contact with the vessel at the injury site,said coating being effective to release an amount of the compound toinhibit restenosis at the site.
 2. The stent of claim 1, wherein thefilaments are poly-l-lactide, and the polymer substrate in the coatingis selected from the group consisting of poly-l-lactide orpoly-dl-lactide.
 3. The stent of claim 1 wherein the drug-releasecoating of poly-lactide polymer is a co-polymer.
 4. The stent of claim 3wherein the co-polymer is comprises poly-lactide and ε-caprolactone. 5.The stent of claim 1, wherein the polymer substrate in the coating isformed of poly-dl-lactide having a thickness between 3-20 microns. 6.The stent of claim 3, which further includes a parylene polymerundercoat having a thickness of between 1-3 microns, disposed betweenthe filaments of the stent body and said poly-lactide coating.
 7. Thestent of claim 3, wherein said coating includes the compound at aninitial concentration of between 50 and 75 weight percent of coating. 8.The stent of claim 1, further comprising a polymer undercoat disposedbetween the filaments of the stent body and said drug-release coating.9. The stent of claim 6, wherein said polymer undercoat is formed of apolymer selected from the group consisting of ethylene vinyl alcohol,parylast, silicone, a fluoropolymer, and parylene.
 10. The stent ofclaim 1, wherein said coating further includes a bioactive agentselected from the group consisting of antiplatelet agents, fibrinolyticagents, and thrombolytic agents.
 11. The stent of claim 1, wherein thepolymer substrate in the coating is formed of poly-dl-lactide.
 12. Thestent of claim 1 wherein R is OH.
 13. The stent of claim 1 wherein thefilaments are metal.
 14. The stent of claim 1 wherein the filaments arebioerodable.
 15. The stent of claim 1 wherein the coating is on theoutside surfaces but not on the inside surfaces of said filaments.